1. Field of the Invention
This invention relates to an improvement in a fluid coking process. It particularly relates to an improvement in the elutriation of the circulating coke in a once-through fluid coking process. The expression "once-through fluid coking" denotes herein a process in which the heavy oil portion of the fluid coking reaction zone effluent is not recycled to the coking zone.
2. Description of the Prior Art
Fluid coking is a well known process which may be carried out with or without recycle of the heavier oil portion of the fluid coking zone effluent. See, for example, U.S. Pat. No. 2,881,130, the teachings of which are hereby incorporated by reference.
Integrated fluid coking and coke gasification processes are also known. See, for example, U.S. Pat. Nos. 3,661,543; 3,816,084, and 4,055,484, the teachings of which are hereby incorporated by reference.
When the fluid coking zone of either the conventional fluid coking process (coking vessel and external heating vessel) or the integrated fluid coking and coke gasification process is conducted as a once-through fluid coking zone, an undesired amount of solid fines of less than about 44 microns in diameter is entrained in the vaporous coker product, which includes normally liquid hydrocarbons. The entrainment of solid fines in the vaporous coker product is greater when the feed to the coking zone is a hydrocarbonaceous oil derived from a coal liquefaction process, including coal liquefaction bottoms, which comprise significant amounts of solids (unconverted coal, ash, etc.).
U.S. Pat. No. 2,865,847 discloses a process for controlling the coke particle size in a fluid coking process.
U.S. Pat. No. 4,055,484 discloses elutriation of coke particles in the riser of a heater of an integrated coking and coke gasification process.
It has now been found that the entrainment of fines of less than about 44 microns in diameter in the coker products can be minimized by removing a substantial portion of these fines prior to recycling a stream of solids to the coking zone.